Fundamental to protection against pathogens is the development of immunologic memory, a dramatically more effective response to previously encountered infections that is largely regulated by helper T lymphocytes (CD4 cells). Little is known about regulation of T-cell memory in part because there is no evidence for distinct precursors of effector and memory T-cells. While many studies have focused on requirements for maintenance of memory, regulation of the induction and re-expression of memory has received little attention. Identifying parameters that control CD4 memory at these stages is essential for exploiting the immune system as a means to protect against disease. Since memory CD4 cells arise from responding naive cells, it is imperative to delineate factors that regulate memory at the initiation of the primary response. The investigator proposes that antigen (Ag) and Ag-presenting cells (APC) play crucial roles in CD4 memory development, and that memory is regulated by selective processes of decreasing Ag concentrations and changing APC requirements during the primary response. Since the dose and form of antigen (Ag) can affect cell mediated vs humoral responses that are associated with Th1 and Th2 cells, respectively, Ag may also play a role in development of subsets of memory CD4 cells. The goal of the proposed studies are: 1) to assess the role of Ag in the development of memory CD4 cells and in the differentiation of Th0, Th1, and Th2 subsets of memory CD4 cells; 2) to evaluate the role of B-cells as APC in the memory CD4 response, both for optimal memory development and for re-eliciting memory responses; and 3) to characterize the memory response in situ in lymphoid tissue to identify anatomical sites where memory responses take place and to determine whether subsets of memory CD4 cells differ in their capacity to promote B-cell expansion, germinal center formation, and isotype switch. The investigator will use both in vitro and in vivo approaches with normal, thymectomized, transgenic, and knockout mice to determine factors that are required for the development and function of memory CD4 cells. The investigator will exploit the response to pigeon cytochrome c (PCC) in H-2K mice which is restricted to the v-alpha-11, v-beta-3 TCR where the investigator can compare responses of normal CD4 cells to those from PCC-specific transgenic mice with the same TCR when the dose and form of Ag (peptide versus naive protein) are varied. The investigator will assess the impact of primary and memory B-cells on CD4 memory using B-cell-deficient mice, hen egg lysozyme Ig transgenic mice, and the B-cell response to the NP hapten which is restricted to the lambda light chain. The proposed studies represents an integrated investigation of factors that will provide essential, basic information about the regulation of CD4 memory.